It is well known that an optical interferometer, e.g. Fabry-Perot, Michelson, Mach-Zehnder types, when properly tuned to the laser frequency, can be used to detect ultrasound and other surface transient motion. One of the present inventors (J-P. Monchalin) has shown in his U.S. Pat. No. 4,659,224--Apr. 21, 1987, that a confocal Fabry-Perot interferometer can be used in the transmission mode for such a detection technique. J-P. Monchalin also describes in his copending U.S. application Ser. No. 07/310,380 filed Feb. 15, 1989, now U.S. Pat. No. 4,966,459 arrangements in which a confocal Fabry-Perot is used in the reflection mode (or sometimes called the optical sideband stripping mode).
In most embodiments described in these two documents and other prior art devices, stabilization of the confocal Fabry-Perot or other interferometer with respect to the laser frequency is performed generally by deriving an additional beam (the stabilization beam) from the output of the laser. This beam is then superimposed colinearly with the beam scattered off the surface and both are sent into the confocal Fabry-Perot. The stabilization ensures that the laser frequency is always located at the most sensitive point on the resonance curve of the interferometer. Polarization optics is generally used to separate these two beams in front of the signal and stabilization detectors. Such an arrangement has the advantage to permit stabilization independently of the reflectivity of the surface but it has the drawback to require to locate the laser in the immediate vicinity of the receiving confocal Fabry-Perot. A solution which could be considered would be to couple this stabilization beam through an optical fiber, but this causes an additional complexity. A convenient setup should therefore provide stabilization by using only the beam scattered off the surface, without a stabilization beam. U.S. Pat. No. 4,659,224 teaches a further embodiment in which no separate stabilization beam is used. However, in that embodiment, the laser beam is phase modulated before it strikes the surface. RF filters are used to separate the stabilization signal from the ultrasound signal.